Thermal transfer printing has displaced impact printing in many applications due to advantages such as the relatively low noise levels which are attained during the printing operation. Thermal transfer printing is widely used in special applications such as in the printing of machine readable bar codes and magnetic alpha-numeric characters. The thermal transfer process provides great flexibility in generating images and allows for broad variations in style, size and color of the printed image. Representative documentation in the area of thermal transfer formulations and thermal transfer media used in thermal transfer printing includes the following patents.
U.S. Pat. No. 3,663,278, issued to J. H. Blose et al. on May 16, 1972, discloses a thermal transfer medium having a coating composition of cellulosic polymer, thermoplastic resin, plasticizer and a "sensible" material such as a dye or pigment.
U.S. Pat. No. 4,315,643, issued to Y. Tokunaga et al. on Feb. 16, 1982, discloses a thermal transfer element comprising a foundation, a color developing layer and a hot melt ink layer. The ink layer includes heat conductive material and a solid wax as a binder material.
U.S. Pat. No. 4,403,224, issued to R. C. Winowski on Sep. 6, 1983, discloses a surface recording layer comprising a resin binder, a pigment dispersed in the binder, and a smudge inhibitor incorporated into and dispersed throughout the surface recording layer, or applied to the surface recording layer as a separate coating.
U.S. Pat. No. 4,687,701, issued to K. Knirsch et al. on Aug. 18, 1987, discloses a heat sensitive inked element using a blend of thermoplastic resins and waxes.
U.S. Pat. No. 4,698,268, issued to S. Ueyama on Oct. 6, 1987, discloses a heat resistant substrate and a heat-sensitive transferring ink layer. An overcoat layer may be formed on the ink layer.
Others include: U.S. Pat. No. 4,707,395; U.S. Pat. No. 4,777,079; U.S. Pat. No. 4,923,749; U.S. Pat. No. 4,988,563; U.S. Pat. No. 5,128,308; and U.S. Pat. No. 5,240,781.
A common feature in these thermal transfer media is the use of a substrate for the ink to be transferred. Polyethylene terephthalate (PET) films are preferred substrates in that the property profile for PET (heat resistance, tensile strength, etc.) is well suited for use in conventional thermal transfer printers. One characteristic of most polymeric films, including PET films, is the generation of static electricity when rolls of these films are unwound. It has been discovered that static electricity from the thermal transfer ribbon can be a source of premature print head wear through static-electrostatic discharge. Therefore, reducing the static level of thermal transfer ribbons is desirable. Adding conductive fillers to non-conductive polymeric materials is known to reduce the static levels of such materials. However, adding such conductive fillers to polyethylene terephthalate is not always possible, particularly where obtained from another source and, furthermore, adding such conductive fillers may detract from the desirable properties of PET film.
The use of separate anti-static layers on films for photographic materials has been disclosed in U.S. Pat. No. 4,916,011. Similar configurations have also been disclosed in U.S. Pat. Nos. 5,079,130 and 5,098,822. These anti-static layers comprise conductive polymers which show a high bonding strength to the substrate. Such a configuration is not advantageous in preparing thermal transfer ribbons in that it requires another coating procedure and may also detract from the desired properties of polyethylene terephthalate during the thermal transfer process. Depending on the position of the anti-static layer (top or bottom), it may either interfere with separation of the ink from the substrate during transfer or affect print head wear.
Employing conductive pigments in the thermal transfer layer of the thermal transfer medium has been found to reduce static levels. However, such conductive fillers may add color to the thermal transfer layer. This may limit the use of such conductive fillers to dark colored ink such as black. It is desirable to reduce the static levels of thermal transfer ribbons without requiring the use of conductive fillers within the thermal transfer layer.
Conductive polymers, i.e., inherently dissipative polymers which do not require conductive fillers, have been found to be suitable replacements for polymers with electroconductive fillers (powdered carbon, powdered nickel, metal particles and the like) for cathodes of electrolytic cells, where the primary function is conductivity. However, conductivity is not the primary function of thermal transfer layers.